Continuous centrifugal tube casting apparatus using a liquid mold

ABSTRACT

A molten castable substance (as metals, glass, plastic, etc.) is continuously centrifugally cast to tube on a centrifuged lining of a heavier liquid mold material (examples of which are lead, tin, and Wood&#39;&#39;s metal). The molten substance is continuously introduced into the starting end of the centrifugal casting machine and forms a molten, axially moving, cylinder on the liquid lining. The substantially solidified tube floats out of the bore at the opposite end of the casting machine on a lining of the liquid mold material and without contacting the solid portions of the machine&#39;&#39;s exit orifice. The unrestricted floating of the tube out of the exit orifice is accomplished by decreasing the diameter of the tube in its molten state by applying a pressure differential between the liquid mold material and the gas internal to the tube being cast. The pressure differential is brought about by various methods as disclosed in the specification. Prior art methods for the continuous centrifugal casting of tube on a liquid mold lining depend on thermal shrinkage of the tube, as it cools in the mold, to permit egress from the machine. Due to the applied pressure differential, such thermal shrinkage to permit unrestricted exit of the tube does not have to be a requirement of the present invention and, as a result, product variability and casting rates can be greatly increased.

United States Patent 1191 Leghorn 1451 Dec. 25, 1973 Filed: Oct. 28,1971 Appl. No.: 193,476

Related U.S. Application Data Continuation-in-part of Ser, No. 768,983,Oct. 21, 1968, Pat. No. 3,616,842, which is a continuation-in-part ofSer. No. 538,506, Feb. 11, 1966, Pat. No. 3,445,922.

U.S. Cl 425/435, 164/64, 164/84 Int. Cl B22d 11/02 Field of Search164/81, 82, 84, 64,

[5 6] References Cited UNITED STATES PATENTS 11/1931 Lindemuth 164/816/1960 Daubersy et al. 164/84 X 5/1969 Leghorn 164/84 X 3,605,859 9/1971Leghorn..... 264/311 X 3,430,680 3/1969 Leghorn 164/81 FOREIGN PATENTSOR APPLICATIONS 22,708 0/1895 Great Britain.... 164/81 7,000,792 7/1970Netherlands 164/81 36 I I 37 aolidified casting Primary Examiner-R.Spencer Annear Attorney-William H. Maxwell [57] ABSTRACT A moltencastable substance (as metals, glass, plastic, etc.) is continuouslycentrifugally cast to tube on a centrifuged lining of a heavier liquidmold material (examples of which are lead, tin, and Woods metal). Themolten substance is continuously introduced into the starting and of thecentrifugal casting machine and forms a molten, axially moving, cylinderon the liquid lining. The substantially solidified tube floats out ofthe bore at the opposite end of the casting machine on a lining of theliquid mold material and without contacting the solid portions of themachines exit orifice. The unrestricted floating of the tube out of theexit orifice is accomplished by decreasing the diameter of the tube inits molten state by applying a pressure differential between the liquidmold material and the gas internal to the tube being cast. The pressuredifferential is brought about by various methods as disclosed in thespecification. Prior art methods for the continuous centrifugal castingof tube on a liquid mold lining depend on thermal shrinkage of the tube,as it cools in the mold, to permit egress from the machine. Due to theapplied pressure differential, such thermal shrinkage to permitunrestricted exit of the tube does not have to be a requirement of thepresent invention and, as a result, product variability and castingrates can be greatly increased.

32 Claims, 22 Drawing Figures PATENTEI] 111-382 51975 3.781.158 SHEET 101- 200 I 600' 1000 I400 I800 5,

330C, 700%, l fie Temperature C.

bxzru Pan e a 721.66 Diameters 2 {ash/all 77115011115} Provided by H Ithe Invention j T i 512,. x L

' T g 6 919 5 5 1 1 raaz'us I 'l l 1 1 1 1 1 1 T /0 so so 0 D J) Parzgcof Tube Diameters A Z (vs. Wall Thickness) 1b which p7'0("55C5 Dependenton jllrz'nk age rbr Exlraaion are Pestrz'atcd PAIENTEUBEEZSIE'H snmanra[III I ///7 PATENTEUU 1 saw u as a II/ III [ll/ll PATENTEUUECZS 191sSHEET 7 0F '8 I/IIII/l INVENTOR. GEORGE 4. L EGHO/Z/V BY Pmmeontcz 15 3,781 158 GEO Q65 1?. LEGHORN CONTINUOUS CENTRIFUGAL TUBE CASTINGAPPARATUS USING A LiQUllD MOLD RELATED APPLICATIONS This application isa continuation-impart of my prior application, Ser. No. 768,983, filedOct. 21, 1968, now 5 U.S. Pat. No. 3,616,842, dated Nov. 2, 1971. Thatapplication was in turn a continuation-in-part of my prior applicationSer. No. 538,506, filed Feb. 1 1, 1966. Said application Ser. No.538,506 became my U.S. Pat. No. 3,445,922. Continuation-impartapplications derived therefrom have become U.S. Pats. No. 3,430,680 and3,605,859.

BACKGROUND OF THE INVENTION It has been shown, in the above-identifiedrelated apl5 plication, that the Maxim process (British Patent No.22,708 of 1875). is limited to the production of cylinders wherein therelation of the cylinders outside diameter to its wall thickness isexpressed by formula 2D=9OT (where D is the OD. of the cylinder and T isits wall thickness) for a low carbon steel exhibiting a phasetransformation at about 600C which causes a slight re-expansion of thecylinders diameter. it has also been shown that production is limited byformula 1D -65T for steels not exhibiting such a phase transformation.This is due to the fact that molten steel, cast upon a liquid leadlining, sinks into the liquid lead by about two-thirds of its wallthickness (the Archimedes principle). Therefore, for a steel tube havinga wall thickness of one inch, the molten steel will sink into the liquidlead by two-thirds inch and the CD. of such a tube will be four-thirdsof an inch greater in diameter than the exit orifice of the Maximcontinuous centrifugal casting machine. The diameter of such a cylindermust, therefore, thermally contract by this amount before the castingcan exit from the machine. The above formulas were derived on this basisand show that, for a one inch Wall thickness, the cast cylinder wouldhave to be at least 65 40 times the wall thickness, or five feet fiveinches in ut diam ert One other patent, Daubersy et al (U.S. Pat. No.2,940,143) concerns the continuous casting of steel tube on alubricating lining of liquid lead. This process also depends on thermalshrinkage of the cast tube, while cooling in its solid state, to achievea diameter small enough to permit its passage through the exit orificeof the casting machine. I

The invention disclosed herein is designed to correct the limitation onproduct output (large diameter cylinders having a fairly thin wallsection) of the Maxim process. It also corrects the casting ratelimitation of the Daubersy et al process and can, in fact, continuouslycast steel tubing at rates of hundreds of tons per hour.

RESUME OF THE INVENTION The present invention provides for introductionof moderate (about 5 to 25 percent by weight of the material being castto tube) to large (over 25 percent by weight of the tube material)amounts of liquid mold material through the casting machine by way ofmaintaining the outside diameter of the cast cylinder less than the l.D.of the exit orifice of the machine. By one method of the invention, theliquid mold material is non-flowing and only minor amounts of liquidmold material need be added to produce the same result. By use of othermethods of the invention, the amounts of liquid mold material requiredcan be increased or decreased at will. By maintaining the CD. of thecast tube less than the l.D. of the exit orifice, the cast tube canfloat out of the casting machine on a lining of liquid mold material.

The methods of the present invention can be used in conjunction withthermal shrinkage, if desired. This is accomplished by creating apressure differential between the liquid mold material, lining the bore(solid container wall) of the centrifugal casting machine, and the gasinternal to the tube being cast. By so doing, the level of the liquidmold material is forced inwardly, and the molten steel cylinder at thestarting end of the casting machine is also forced inwardly to a smallerdiame' ter. The diameter of the molten tube can be equalized at anypredetermined size depending on a precalculated pressure differential.

My prior U.S. Pat. No. 3,616,842 shows the means by which suchcalculations are made and gives specific examples thereof.

it should be noted that the molten tube material will sink into theliquid mold material to a level where it has displaced its own weight ofthe liquid mold material as required by the Archimedes principle. The0.1). of the molten tube is decreased to a size that permitsunrestricted egress through the exit orifice of the casting machine by aprecalculated pressure differential that forces the liquid molt materialinwardly, thus constricting the molten tubes diameter. The molten tubethus partially or completely solidifies to a diameter that is less thanthat of the 1.1). of the exit orifice as required for the tube to floatfreely out of the bore of the casting machine.

For purposes of clarity, the following specific example, which wasoriginally disclosed in related application Ser. No. 768,983, ispresented. in the presented example, the CD. of the tube in its moltenstate is made equal to the l.D. of the exit orifice. This is forpurposes of calculation only. in ordinary casting practice, the OD. ofthe molten tube can be greater than the l.D. of the exit orifice whenallowance is made for thermal shrinkage of the tube, in its solidifiedstate, to reduce its O.D. to less than the l.D. of the exit orifice. inthis instance, an external means for slowing down the tubes exit rate isused to make the tubes O.D. smaller than the exit orifice [.D. at apoint considerably to the rear of the exit orifice.

1n the example case, the exit orifice l.D. of the casting machine is 10inches, and this is also the ID. of the centrifuged cylinder of liquidmold material lining the solid wall (container) prior to introduction ofthe molten steel to be cast to tube. in this specific instance, a moltentube of mild steel,which without an applied pressure differential wouldsink into the liquid mold material by two-thirds of an inch on theradius, is used. The wall thickness would, therefore, be one inch.Calculations are predicated on the tubes final O.D., after the appliedpressure differential has reduced its initial required 10 inches O.D.since the introduced error is relatively small.

The molten steel tube, after application of the pressure differential,has an CD. of 10 inches and and l.D. of 8 inches (wall thickness ofabout 1 inch) and is centrifugally cast at a rotational speed which isequivalent to 50 Gs (gravities). At the solidification temperature of1500C, the density of the just-solidifying steel is 7.30 g/cc or 0.264lbs/cu.inch. However. since the casting operation is being carried outat 50 G's, the effective weight of a cubic inch of the steel (at l500C)is 50 X' 0.264 or 13.2 pounds. But, if we project a square inch area ofsurface on the CD. of the tube onto the tubes axis (see FIG. 3), we havea truncated wedge removed from the tube wall which has an exteriorsurface area of one sq. in. and an interior surface area of 0.8 sq. in.,along with a radial (wall thickness) depth ofone inch. The volume ofthis truncated wedge is 0.9 cubic inches, and this volume of moltenmetal bears on the one square inch of outer surface due to thecentrifugal action. Since one cubic inch of the metal weighs 13.2pounds, then 0.9 cubic inches of the truncated wedge will have aneffective weight of 0.9 X 13.2 or 11.9 pounds at 50 G's, and this weightis exerted against the one square inch area on the OD. surface andexerts a pressure of 11.9 psi.

Therefore, in order to raise the liquid level of the mold materialinwardly and thus bring the CD. of the molten metal tube to ten inches,a pressure differential, between the liquid mold material and the gasinternal to the tube, of 11.9 psi must be achieved. The molten tubemetal still sinks into the liquid mold material by two-thirds of an inchon the radius, but the level of the liquid mold material is raisedinwardly by this amount by the l 1.9 psi pressure differential appliedby the various methods of the invention.

It should be noted that the Archimedes principle states that a floatingbody will sink into the liquid on which it floats to a point where itdisplaces its own weight of the liquid. At 50 G's, the floating bodyeffectively weighs 50 times as much, but so does the liquid which isdisplaced. Therefore, the sinkage is the same regardless of the Gfactor.

The necessary pressure differential can, and is, created by any one orany combination or permutation of the following five species of mymethods (which will herein be designated as Method 1, Method 2,...Method5 b elovv iandwhen referenced in the teachings of this disclosure.

Method 1. By restricting the liquid mold exit orifice (the annular gapbetween the solidified tube 0. D. and the centrifuge's exit orifice l.D.),

Method 2. By extending the length of the weir (exit orifice) lip to asufficiently great extent that the required downstream line pressuredrop of l 1.9 psi is'experienced.

Method 3. By creating a vacuum within the cast tube that offsets thesinkage ofthe 1 inch thick layer of steel.

Method 4. By raising the atmospheric (gas) pressure (exterior to thetube and the exit orifice or at the entrance end and exterior to anyvacuum seal means) by the desired amount over that of the ambientatmospheric pressure (14.7 p.s.i. is the average sea level atmosphericpressure'and 14.7 11.9 or 26.6 p.s.i. would be required under normalconditions).

Method 5. Instead of a gas pressure differential of 1 1.9 p.s.i. (totalof 26.6 p.s.i. where the interior of the tube is at 14.7 p.s.i. ambientpressure) being applied at the enclosure at the exit end as in Method 4,a liquid pressure can be exerted on the liquid mold material (as by asuitable pump or a head of liquid mold material supplied by an overheadtundish or reservoir). By exerting this liquid pressure at the exit end,the liquid mold material will be forced inwardly and constrict themolten tube to the desired diameter. In this case, the bed of liquidmold material will be substantially nonflowing. 1n the case where theextra liquid mold material pressure is applied at the exit end andoverflows a fixed-diameter weir at the entrance end (the l. D. of suchan entrance end overflow weir would be approximately 10 inches minus 4/3inches or 8 2/3 inches in the example case of a 10 inches O.D. tube and1 inch wall thickness at 50 GS), the liquid mold material will flowcountercurrent to the axial movement of the tube being continuouslycast. Such countercurrent flow is optimum for heat extraction purposes.

Method 5 can also be applied at the starting end (tube casting) of themachine in much the same way of Method 4, and with the same advantage ofgreater throughput of liquid mold material for heat extracting purposes.Also, both Methods 5 and 4 can be applied simultaneously at both thestarting and exit ends of the machine for purposes of maximizing thepressure differential necessary for reducing the OD. ofa very thick wallof heavy tube. Also, Method 5 can be applied at any intermediate point(as at the mid-length) of the casting machine or at a multiplicity ofsuch points if so desired.

Additionally, Methods 5 and 4 can be applied in series at the ends ofthe casting machine in order to facilitate the pressure buildup of theliquid mold material.

With respect to Methods 1 and 2, it can be noted that these methods areentirely feasible. However, a considerable amount of liquid moldmaterial must be introduced into the system to maintain the desiredpressure differential (back-pressure in the case where Methods 1 and/or2 only are used) at a desired equilibrium value, although this can varyover a wide range depending on whether the cast tube's exit iscontrolled by the fluid friction inherent to Methods 1 and 2, or by abraking means (as by magnetic field braking or an external mechanicalmeans such as shown in the Maxim process or other, such as the suctionof the internal vacuum when Methods 1 and 2 are used in conjunction withMethod 3), which slows down the axial movement of the cast tube.

Method 3, the creation ofa partial vacuum onv the interior of the tube,is a forceful means of accomplishing the desired pressure differentialand in-troduces other beneficial effects as well.

This system exhibits the following advantages:

a. After an initial purge with an inert gas, at start-up, and thenapplying the suction, the gases given off bythe molten metal are of areducing or inert nature (as carbon-monoxide, hydrogen and nitrogen),and these gases maintain the inner surfaces ofthe tube in a brightoxide-free condition which permits and facilitates the pressure-weldingofthe contiguous interior surfaces of the tube one to the other.

b. The primary advantage is the reduction of the cast tube 0D. to apoint where it is less than the exit orifice ID. of the centrifugalcaster.

c. The molten material (being cast to tube) is effectively degassed bythe internal vacuum during its entrance into the centrifuge via aconduit extending through and sealed to the nonrotating seal plate. As amatter of note, it is preferred to vacuum-degas molten casting metalprior to its introduction into the continuous centrifugal tube castingdevice so as to cut down the amount of gas given off by the moltenmetal.

(I. The liquid mold material has less chance of oxidation since no airis internal to the casting chamber.

e. The internal partial vacuum materially aids the collapse formingoperation.

f. The internal partial pressure of reducing gases can be maintainedinterior to the tube, as has been revealed in the teachings related U.S.Pat. No. 3,616,842, for as long as desired.

In the Method 4, the volume external to the centrifuge is enclosed toafford an effective seal which permits the application of a gas pressurewhich forces the liquid molt material to back up in the centrifugalcaster until it attains the desired ID. This pressurization isaccomplished with a dry, inert gas, such as nitrogen, argon, helium, orthe like.

Method 4 has the further advantage of preventing any oxidation of theliquid mold material (as lead, leadtin, etc.) since the liquid moldmaterial is protected by the inert gas of the external enclosure. Also,the higher than ambient pressure of the inert gas helps to suppress thevaporizing tendency of the liquid mold at the exit or overflow-end ofthe centrifuge.

Method 5 exhibits the following advantages.

a. Both the non-flowing and countercurrent flow of liquid mold materialcreate a natural hot zone at the starting (tube casting) end of themachine, and such a hot zone greatly facilitates the leveling andsmoothing of the molten cylinder and the solidified product.

b. It is the most foreceful means for reducing the CD. of the tube inits molten state to the desired diameter for free exit. Very heavy wallsections (which may encompass the entire radius of the cast tube) can becast by this means.

c. Countercurrent flow is recognized as the most efficient means of heatextraction.

d. Very large amounts of liquid mold material maybe forced through themachine for heat extraction, if so desired, by the'various systems ofthe method.

e. By means of the head or liquid level of the liquid mold material inthe supplying overhead reservoir, or standpipe which shows the appliedhead for any pumping system, the exact pressure of the liquid moldmaterial can be determined and adjusted. It affords a good means ofliquid level control within the machine.

From the foregoing advantages of the various methods of the invention,it can readily be seen that all methods can be preferred under specificcircumstances.

It should be noted that batch-type centrifugal casting is an old andwell-established art. Such parameters as the rotational speed necessaryto produce a specific G force for a specific mold diameter are wellknown as, also, are the lower and upper practical limits of G forces(rotational speeds) utilized. It is sufficient to note herein that thesupporting action of the liquid mold material on the outer surface ofthe tube being centrifugally cast (and, also, the use of Methods 3, 4and 5, or combinations thereof) permits the use of much higherrotational speeds (G forces) than is permissable with a conventionaldry-wall centrifugal mold.

With respect to the Methods 3, 4 and 5, it is preferred to utilizehigher internal vacuums (Method 3) and lower external positive pressures(Methods 4 and 5) where tubes having a smaller diameter and heavier wallthickness are concerned. Conversely, in the production of large diametertubes of thinner wall section, it'is preferred to utilize a much lowerinternal vacuum (Method 3) and higher external positive pressures(Methods 4 and 5) in combination. The reason for this preference is thatthe ambient pressure on the tube which is directly proportional to thecross-sectional area of the tube and, also, to the pressure differentialbetween the ambient atmospheric pressure and the internal vacuum. As anexample, a tube having a 10 inch O.D. (cross-sectional area of 78.5 sq.inches) and an internal vacuum of 4.7 psi (pressure differential of 14.74.7 10 psi with regards to a standard atmospheric pressure) wouldexperience a backward thrust of 78.5 in. 2 X 10 psi or 78 5 pounds. Inother words, it would require a force of 785 pounds on the tube tocounteract the internal suction and pull the tube out of the bore of thecentrifugal casting machine. On the other hand, a large diameterthin-walled tube (30 inches in outside diameter as an example) wouldhave a cross-sectional area of 709 sq. inches, and, if the pressuredifferential (between the interior vacuum and the ambient pressure) was10 psi, a force of 7090 pounds would be required to get the tube out ofthe bore of the casting machine. If the 30 inch diameter tube had a Ainch wall thickness and was centrifugally cast at 50 Gs, the pressuredifferential necessary to counterbalance the steel would be one-fourthof 13.2 psi or 3.3 psi. In this case, the required 3.3 psi could be madeup entirely by application of a positive external pressure (Methods 4and 5) of 14.7 3.3 or 18 psi, and the internal pressure of the 30 inchesdiameter tube would be 14.7 psi or the same as the ambient pressure. Bythis technique, a very small force (supplied by the head of moltensteel) would be required to extract the tube from the bore of thecasting machine since the external pressure (of Methods 4 and 5) acts onthe periphery of the tube to just counterbalance the weight of the steeltube at 50 Gs and does not act on the end (cross-sectional area) of thetube to create a backforce which must be overcome (as in Method 3) toget the tube out of the casters bore.

With respect to the foregoing example, it can be appreciated that ahigher than atmospheric pressure can be utilized internal to the tubebeing cast to aid in forcing the tube out of the bore of the castingmachine. Also, a partial vacuum within the tube being cast can be usedas a means of controlling the exit rate of the cast tube from thecasting machine since, in the case of an internal vacuum, the exit ratecan be retarded by the applied suction. In the example of a largediameter thin walled tube (30 inches in OD.) having 709 square inches ofcross-sectional area and a A inch wall thickness at 50 G5 (a pressuredifferential of 3.3 psi to counterbalance the steel), a pressure of 15.7psi could be used internal to the tube (one pound gage pressure aboveatmospheric) and this would cause an outward thrust on the end of thetube of 709 pounds. A restraining mechanism of any type would be used tocontrol the exit rate in such a case. The pressure differential of 3.3

psi could then be made up of a positive external pressure (greater thanthat of the surrounding atmosphere) of 3.3 1 14.7 or 19 psi or 4.3 psiabove atmospheric and this can be accomplished by Method 4 and/or Method5.

lt is readily apparent from the foregoing examples that a very widerange of latitude is available to the operator, in the application of aninternal vacuum (Method 3) and an external positive pressure (Methods 4and S), for ready extraction of a tube from the centrifugal castingmachine A judicious (readily calculated) selection of internal andexternal pressures is available for all practical casting requirements.

All of the foregoing examples have been predicated on the use of aninternal vacuum (Method 3), or an external positive pressure (Methods 4and or a combination thereof, just counterbalancing the centrifugalweight of the layer of metal being cast and, under these circumstances,any slight thermal contraction (as in cooling from l500C solidificationtemperature of mild steel down to a collapse deforming and roll-weldingtemperature of about lll5C) or slight back-pressure (as is normallyattendant to such a system by Method 1 and/or Method 2) is sufficientfor free exit of the tube from the exit orifice.

Actually, by increasing the through-put of liquid mold material for anyfixed conditions of Methods 1 and/or 2, such back-pressure quicklyasserts itself, and the molten part of the tube being cast issqueezed-in to a decreased equilibrium O.D. Due to this combined action,of Method 1 and/or 2 in combination with Methods 3, 4 and 5, the actionof Methods 3, 4 and 5 can be considerably less than that necessary tomake the CD. of the cast tube less than the [.D. of the exit orifice ofthe centrifugal casting machine. The action of Methods l and/or 2 can beutilized to further decrease the OD. of the cast tube to the amountdesired for purposes of exit from the system.

In the case where the pressure differential of Method 3 and/or 4 issufficiently great to more than just counterbalance the centrifugalweight of the material being cast, it might be expected that the tubewould decrease in diameter (which it does) to the extent that it wouldlift away from the liquid mold and permit ingress of air or inert gasinto the vacuum of the tubes interior via bubbling through the moltenzone of the tube. This can and does happen, but not immediately beyondthe point where the pressure differential overbalances the zero-point.

A stable-state condition exits for pressure differentials in excess ofthe zero-point, and this is due to the wetting action (attraction) ofthe liquid mold material (especially where tin is present) and thesurface tension of the molten material being so cast. This operatingarea(pressure differential beyond the zero-point) is not actually used sincethe stable-state condition is not that broad and can readily bedestroyed by any out-ofbalance or other vibration-producing condition ofthe rotating system. It does, however, afford a usable margin of safetyfor the condition of exact counterbalance.

However, Method 3 and/or Method 4 may be used by taking advantage ofthis stable state condition, to permit the tube to exit, from thecasting machines exit orifice 9, in a molten state provided that theconditions of exact counterbalance are closely approximated and themolten tubes 0D. is very close to its solidification temperature. Inthis case, the molten tube, as soon as it departs from the liquid moldmaterial at the exit end of the centrifuge, is immediately congealed bya great multiplicity of cooling spray jets which create an inwardpressure on the CD. of the molten tube and rapidly solidify it. Suchcooling sprays can be composed of suspensions of hydrocarbons (as anemulsion of oil in water) in water or solutions of water and variousalcohols which act as a reductant and prevent substantially theoxidation of the exiting liquid mold material. Such a means for castingthe tube is non-preferred since the conditions for successfully carryingout the process are more exacting than the other preferred systems ofthe invention. However, it has the advantage of permitting use of ashorter casting apparatus or one of a moderate length with increasedrate of casting.

The danger of gas being forced into the interior of the tube viabubbling through the molten tube material (as can occur with thegas-induced pressure differential of Method 4) does not exist whereMethod 5 is used to create the pressure differential. Since Method 5uses a pressurized liquid mold material at the exit orifice of thecasting machine, any excess pressure will merely decrease the diameterof the tube being cast. Due to this safer action, Method 5 is usuallypreferred over Method 4.

It is one of the important features of this invention to utilize theadvantageous system of a vacuum internal to the tube being cast (Method3) in the instance wherein tube itself is the end-item, instead of alongitudinal structure formed by inwardly collapsing the tube walls overits entire output length as taught in parent patent US. Pat. No.3,445,922. In the practice of making tube for its own use, a tube(having a capped or crimped vacuum sealed exit end) is used as thestarting tube so that the desired vacuum (depending on the wallthickness of the tube, the densities of the molten tube metal and theliquid mold, the G force of the centrifuge, and the ambient pressure ofthe atmosphere) can be drawn on the tube interior. The machine thencontinuously produces a long length of solidified rotating tube whichexits into an axially aligned cradle which permits such combined egressand rotation. Such a cradle can rotate with the tube by virtue of thesame drive mechanism as that which rotates the centrifugal castingmachine. A multiplicity of axially aligned rollers supports theperiphery of the tube and, at the same time, can either permit or causethe tube to move axially away from the casting machine. In the casewhere axial movement is permitted, the rollers are mere idlers which areattached to and rotate with the cradle. In the case where they cause thetube to move axially, the rollers are spring or piston loaded onto theouter surface of the tube to give a friction drive contact which pullsthe tube from the bore of the centrifuge as is necessary where aninternal vacuum (Method 3), which causes a suction, must be opposed. Therollers, in this instance, are suitably driven by sun gears (via asuitable gear cluster system for such power transmission) and areactivated or de-activated by a suitable clutch mechanism. Suchmechanisms are well known to those practiced in the art of rotarycoupling and un-coupling. At the same time, there is an axial gap inthis cradle system, near the exit end of the centrifuge, withappropriate torch reheating means and rotating opposed swaging orforging hammers which move in axial synchronization with the exitingtube and swages or pinches a re-heated section of the tube to avacuum-tight closure after any desired length has been produced. Thepinch or swage closing mechanism then returns to the initial startingplace where its operation is re-commenced after another appropriatelength of vacuum-sealed tube has been produced. Along with the swagingmechanism, and axially further away from the centrifugal caster by anyappropriate length (a two-foot long swaged section and a ZOO-foot lengthof tube between swages would limit the loss of tube due to swaging toone percent), is located an appropriate cutoff device which travels inaxial synchronism with the exiting tube and cuts off the tube at themiddle of the swaged or forged-down closure so as not to destroy theintegrity of the internal vacuum. After cutting the tube in the axialcenter of the swaged section, the cutoff returns to its starting pointfor recoupling to the axial travel mechanism and cut-off of the tubesection at the appropriate time. By this synchronized and discretelyrepeatable sequence of swaging-down and subsequently cutting off theexiting tube, the integrity of the internal vacuum (with its manifoldadvantages) is maintained during and after the tube casting operation.

It is convenient to forge-flatten the exiting tube (just as a soda-strawcan be pinch-flattened in a selected area between thum and forefinger)at the separating point. However, even though this serves as a simplemeans of sealing and maintaining the integrity of the internal vacuum,it is the preferred method of this invention to swage or peripherallyhammer forge such separation points to a solid round having its forgewelded center-line coincident with the axis of the tube. These endclosures (after separation of the tube lengths at the mid-length of thesolid swaged-down closure) can be cut from the tube'ends with anintegral portion of the tube length as long as desired. Such cut-offclosure lengths are conveniently used to fabricate pressure bottles ortanks for oxy-acetylene, propane storage and the like. In this manner,the closure part of the tube is not subject to re-melt, but affordsgreat economy in the manufacture of pressure tanks and storage vehicles.

My preferred means for extracting (pulling the tube out of the bore ofthe caster in opposition to the suction of the internal vacuum) is topower the rotating swaging apparatus so that, once it has swaged downthe tube to a vacuum-tight solid round, the swaging apparatus remainsgripped to the solid reduced tube closure and pulls the tube out of thebore. The axial travel of the apparatus can be powered by any convenientmeans (such as a chain drive, cog-wheel, worm screw, etc.) and can begeared to or be separate from the rotational means as desired. Thesystem utilizes two such swaging down and pull-out mechanisms so that,while one mechanism is pulling out the tube, the second mechanism can beswaging-down a tube closure some 200 feet closer to the centrifugalcaster. Once the second mechanism has swaged-down and gripped the tubeclosure for powered pull-out, the first mechanism (axially further awayfrom the centrifugal caster) then severs the tube lengths from eachother at the mid-length of the swaged-down closure so as not to destroythe vacuum seal. The first mechanism is then returned to the startingpoint to restart as the second mechanism. The two mechanisms thuscontinually replace each other at the starting point.

In the foregoing manner, long sections of tube (like straight sausagelinks) are produced which have an internal vacuum of a partial nature.

A great advantage of continuous centrifugal casting derives from thefact that small producers, who may have only as much as 100 tons ofmelting capacity, can

effectively compete with the larger steel producers due to the advantageof strategic location. As an example, a ton heat of steel can becontinuously centrifugally cast into a tube having a three-foot diameterand a half inch wall thickness, of one continuous length generallyexceeding a thousand feet. This is accomplished by capping the end ofthe starter tube, as by welding, and then producing the thousand footlength of tube by one pour of the one hundred ton melt. Such tube can beconverted to pipeline or be collapseformed to a plate one inch thick andabout five feet wide. in such a case, the apparatus remains running atthe end of pour until the molten steel within the machine has solidifiedinto a solid tube, the external length of produced tube is severed nearthe exit end of the machine, and the solidifed portion within themachine is utilized as the starter blank for the next one hundred tonheat of steel.

lt is a purpose of this invention to improve the invention of Maxim (Br.Patent No. 22,708) by application thereto of Method 3 (a vacuum internalto the tube being cast) or Methods 4 and 5 (a positive external pressureexterior to the tube and the exit orifice or at the entrance of thecentrifuge) and combinations of Methods 3 and 4 or 5.

In the Maxim process, as improved by the foregoing means, a static (notaxially flowing) centrifuged cylinder of liquid mold material has itsinterior diameter (adjacent to the exit orifice annular weir)substantially equal to the ID. of the exit orifice of the centrifuge. Noliquid mold material overflows the exit orifice weir except the dragoutthat naturally occurs with the Maxim process. Small additions of liquidmold material are added to the system by any convenient means so as tocontinually make up the liquid level and compensate for any losses dueto drag-out, vaporization, etc. The application of Methods 3, and 4 or5, as taught in this inventions disclosure, may be utilized to decreasethe CD. of the semi-solidifed tube (being cast) to a slight extent, orto its greatest possible extent, or to any inbetween extent as desired.Due to the Maxim process not having available an exiting volume ofliquid mold material which can be restricted to build up an aidingback-pressure by the restriction to flow Methods of l and 2, the presentinvention must depend to a slight or a large extent (depending on theamount of application of the Methods of 3 and/or 4) on the diametricalshrinkage of the tube O.D. as it cools to the desired exit temperature.The exiting rate of the tube is controlled, as in the Maxim process, sothat the OD. of the tube thermally shrinks to a less value than the ID.of the exit orifice considerably prior to passage through the annularexit orifice in order to preclude jamming. However, by using Method 5,such a backpressure is established, and the OD. of the cast tube can bereadily restricted to one which is less then the ID. of the exitorifice. Method 5 obviates the necessity for thermal shrinkage to aid indecreasing the tubes diameter.

The Methods of 3, 4 and 5, or combinations thereof, are also applied asan improvement to the process of Daubersy et al (U. S. Pat. No.2,940,143) as a positive and practical means of reducing the CD. of thetube (being cast) to one which is less than the exit orifice ID. Theamount of application of the above Methods extends from the minimum tothe maximum range as desired. By this means, small amounts of liquidmold material (normally less than 5 percent of the throughput weight ofthe molten metal being cast to tube on a timed basis) may becontinuously circulated through the system in order to maintain a ringof liquid mold material between the outside surface of the tube and theface of the annular exit orifice of the centrifuge. I also apply, as animprovement to the Daubersy et al process, the means elucidated hereinfor the positive extraction of the centrifugally cast tube so that acontrolled rate of output can be effected and thus preclude the dangerof jamming the tube into the exit orifice of the casting machines. Ialso apply, to the teachings of Maxim and Daubersy, the methods ofvacuum sealing at the exiting end of the tube so that Method 3 can beeffectively applied to these older processes.

Whereas Methods 1 and 2 are inherent to the Daubersy et al invention,their use is restricted to reducing the CD. of the cast tube in itssolidified, but still plastic, state. Also, the CD. of the hotsolidified tube is maintained larger than the ID. of the exit orifice sothat an interference fit can occur with the lip of the exit orifice byway of preventing the tube s escape from the casting machine until itsthermal shrinkage has been accomplished. This is the Daubersy et almeans of controlled output. In the present invention, Methods 1 and 2are utilized to reduce the CD. of the cast tube in its molten state andto reduce the CD. of the tube to one that is less than the ID. of theexit orifice at a point that is considerably to the rear of saidorifice. In this manner, the jamming of the solified tube into the exitorifice (as required in the Daubersy et a] process) is precluded, andthe casting machine can be made as long as desired for continued heatextraction and tube solidification. Much higher rates of output are thuspermitted.

The liquid mold material, used in conjunction with the continuouscentrifugal casting systems herein disclosed, embodies the folowingcharacteristics: (1) has a solidification temperature lower than that ofthe material being cast; (2) is substantially immiscible with andnon-reactive to the molten material being cast (except where alloying isdesired for a corrosionpreventive surface coating such as tin or iron);(3) has a boiling point which is substantially higher than the meltingpoint of the material being cast under the rotational forces involved(high G rotation suppresses the boiling tendency); and (4) has a densitywhich is greater than the material being cast to tube. Many othersubstances (such as bismuth, indium, silver, etc.) and mixtures thereofcan be used as liquid mold materials. The preferred substances are lead,tin, and mixtures thereof.

The novel features which are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, both as to its organization and method of operation, as well asadditional objects and advantages thereof, will best be understood fromthe description when read in connection with the accompanying drawings.

IN THE DRAWINGS FIG. 1 is a graphical representation of the change inspecific volume of a solidifying and cooling steel;

FIG. 2 is a graphical representation of the example formulas l and 2,respectively, which show the limitations of product output of liquidmold centrifugal tube casting machines which depend on diametricalshrinkage of the solidified tube to accomplish extraction thereof;

FIG. 3 is a diagram of a unit volume section of tubing wall, in the formofa truncated wedge with radial sides, used in computing the pressuredifferential required for reducing the CD. of the tube being cast to onethat is less than the I.D. of the exit orifice;

FIG. 4 is a partial sectional view of a simplified centrifugal,liquid-mold continuous casting machine wherein no exit orifice lip(reduced diameter annular orifice weir) is used;

FIG. 5 is a more detailed axial sectional view of a liquid-moldcontinuous centrifugal casting machine adapted to the floating of tubeout of the bore;

FIG. 6 is an axial sectional view of one embodiment of this inventiondepicting vacuum sealing means at the entrance (pouring) end of thecentrifuge and a means of vacuum sealing the tube subsequent to the exitend;

FIGS. 6A, 6B and 6C are partial axial sectional views depicting otherembodiments of the entrance end vacuum sealing means;

FIG. 7 is an axial sectional view of an embodiment of the exit end of acentrifugal casting machine which depicts means of enclosure to effect apositive pressure (above ambient) external to the exiting tube;

FIGS. 8, 8A and 8B are partial axial sectional views depicting variousmeans of layering the molten metal onto the liquid mold material in asmooth continuous manner;

FIGS. 9, 9A and 9B are partial axial sectional views depicting variousmeans whereby the liquid mold material may be introduced or pressurizedat the exit end of the casting machine or at points intermediate betweenthe exit and starting ends (Method 5);

FIGS. 10, 10A and 10B are partial sectional axial views depictingvarious means at the starting end of the casting machine when the liquidmold material is pressurized by the arrangements shown in FIGS. 9(Method FIG. 1 l is a partial axial sectional view ofa system forpressurizing the liquid mold material at the starting end of the castingmachine by Method 5;

FIG. 12 is an axial sectional view of a simplified casting machinewhereby tube is cast vertically downwards on a lining of liquid moldmaterial; and

FIG. 13 is a simplified sectional view of an inverted modification ofFIG. 12.

DETAILED DESCRIPTION Referring now to the drawings in detail, and inparticular to FIG. 1 (re-drawn from Wulff's Metallurgy for Engineers), lhave shown, by way of example, that a centrifugally cast mild steel tubewill experience a diametrical shrinkage of about 2.0 percent in coolingfrom the solidification temperature of about 1500C to a temperature justabove that of the melting point of a liquid lead mold material, or 330C.I have also shown that the diametrical shrinkage of a centrifugally castmild steel tube in cooling from l500C down to 700C is about 1.53percent.

By using these percent shrinkage values and the densities of the axiallyflowing centrifuged tube of mild steel and the liquid lead mold at thetemperatures involved, I have derived Formulas l and 2, given earlier,which illustrate the minimum diameter of a mild steel tube for any givenwall thickness, in order to satisfy the displacement requirements of theArchimedes principle and the diametrical contraction requirements forwithdrawal of the tube from the exit orifice of the centrifugal castingmachine where such tube shrinkage is the means by which such exit isaccomplished.

The limitations of Formulas l and 2 (D=65T and D=9OT, respectively) aregraphically illustrated in FIG. 2 wherein, for any wall thickness ofmild steel being centrifugally cast to tube on a liquid mold of lead,the tube diameter necessary to permit sufficient contraction of the tubeso that it can just escape out of the systems exit orifice, can readilybe determined. it should be realized that these are merely exampleformulas and graphical figures which are applicable to the continuouscentrifugal casting of a mild steel tube on a liquid lead mold. Similarformulas and graphs can readily be derived for other systems of castingmaterials (as aluminum, glass, ceramics, copper, nickel, plastics, etc.)when used in conjunction with other liquid mold materials (as thepreferred lead, tin, and lead-tin alloys).

Reference is now made to FIG. 4 which is an axial cross-sectional viewof a simplified version of a continuous centrifugal tube caster (castingmachine) or centrifuge utilizing a liquid mold and having an exitorifice diameter which is greater than the CD. of the cast tube. In FIG.4!, the centrifugal caster is rotatable about its axis 1 by means ofsuitable trunnions and drive mechanisms not shown. At the entranceorifice 2 a liquid mold material 3 is poured upon the rotating annularrefractory and thermally insulating part 4 of the centrifuge via spout5. (It can also be introduced as an addition to the molten tubematerial.) At the same time, the molten material 6 to be cast to tube,is poured onto the refractory part 4, of the centrifuge by way of spout7. The refractory part 4 of the centrifuge extends to a point 8 (towardsthe exit end 9) so as to form a hotzone 10 wherein solidification of thetube is retarded and where the molten tube material 6 and the liwuidmold material 3 have time to layer into over-and-underlying cylindricalshells in the liquid state. The refractory part 4 is enclosed in astructural shell M which supports the refractory part t and then extendsto the exit end 9 as the solid wall 12 of the centrifuge. The solid wall)12 is cooled on its exterior surface by multiple peripherally arrangedjets of water (not shown) or other cooling material so as to remove heatfrom the molten tube material 6 through the liquid mold lining 22 andsolidify the molten material to a solid tube 14. The solidified tube lidcontinues out of the centrifuge into an axially aligned and rotatingcradle (not shown) and is intennittently cut off to desired lengths byany desired mechanism, such as that shown in the Maxim patent (Br.22,708). The liquid mold material 3 cascades at from the annular exitend 9 of the centrifuge into an annular trough (not shown), such as thatused in U.S. Pat. No. 2,866,703, issued to Gross in 1958, wherein anaxially flowing molten metal effluent is spun out of the exit end of acentrifuge into an annular catch basin. The liquid mold material 3 isthen recirculated back to the pouring spout 5 by any convenient means,such as that of U.S. Pat. No. 2,617,148, issued to Ryan in 1952, whereina metallic liquid mold material is recirculated from the exit end of acasting machine back to the entrance end via suitable heat exchangers(coolers) and a suitable pump.

it should be noted that the continuous centrifugal tube casting machineof FIG. 4 utilizes a long bore and a limited amount of liquid moldmaterial addition so as lid to accentuate the shearing action(resistance flow) in the liquid mold material. The bore of the castingma chine can be extended to the point that heat extraction through thesolid walls 12 of the casting machine is sufficient to solidify most ofthe liquid mold material 3 near the exit end 9. In this manner, theresistance to flow of the liquid mold material is accentuated (Method1). This simplified type of casting machine (having no exit orifice lipor a built-up lip of solidified liquid mold material) exhibitscontrolled output of the cast tube by its very long length since such alength resists the movement of the tube axially along its bore. It hasthe drawback of requiring an inconveniently long mold to permit the tubematerial to solidify prior to exit. However, it can be used with anyconvenient other means to restrict the exit rate of the cast tube andpermit solidification thereof in a machine of moderate length.

The refractory part iof the centrifuge is preferably made of pyrolyticboron nitride or pyrolytic graphite with the C" planes (the plane of lowheat conductivity) being perpendicular to the axis 1 of the bore and theA plane (the plane of greatest heat conductivity) being parallel to theaxis of the bore. in this manner, the inside (l.D.) of the hotzone Ml isat a high and uniform heat that prevents solidification in that area. Bygreatly extending such a hot zone, an extended hot zone results whichpermits the accentuation of gravity segregation to a useful extent.

The use of an extended hot zone and the benefits which can be made toaccrue in certain cases (such as the manufacture of automotive sheetsteel) are discussed in my prior U.S. Pat. No. 3,616,842.

This system has the virtue of extreme simplicity; however, due to thehigh G forces involved, the liquid mold material can have a higherexiting flow than the cast tube with (if used) controlled pull-out. Thisflow differential can cause wrinkling (shirt-sleeving) of the tubesurface at the point of incipient solidification, and this surfaceroughness anchors the liquid mold material and results in excessivedrag-out.

FIG. 5 is illustrative of a more sophisticated system for the continuouscentrifugal casting of tube on an axially flowing lining of liquid moldmaterial. A criterion of the apparatus of FIG. 5 is that the OD. of thetube (prior to the point of exit) be less than the exit orifice ll). inFIG. 5, the molten tube material 6 pours into an annular trough 16 whichis similar to the annular distributing chamber used by Stravs and J agerin U.S. Pat. No. 777,559 of 1904 and serves to take up the impact of theinpouring molten material 6 and to evenly distribute it, via therefractory annular shelf 117, as a molten cylindrical tube within thebore of the centrifuge. The refractory part 4- of the centrifuge isextended towards the exit end 9, as shown, so as to form a hot zone It)whereon the cylinder of molten tube material 6 becomes leveled orlayered into a smooth cylindrical tube 26 on top of a thin cylindricallayer 118 of hot liquid mold material.

The liquid mold material 3 is poured into an annular sump 19 and moves(via a multiplicity of longitudinal holes 20 peripherally spaced aroundthe base of the refractory part 4) downstream in the centrifugal castervia the main series of flow-holes 2b to the main exit 2t whereat themain part of the cooler liquid mold material flows into aheat-extracting ring 22 of liquid mold material which both supports andsolidifies the ring of molten material to an exiting solid tube 14.Upstream from the main exit 21 of the liquid mold material is anotherseries of annular liquid mold flow-holes 23 via which a restricted(quite small) amount of liquid mold material forms a thin lining 18 ofvery hot liquid mold material which extends downstream for the length ofthe hot-zone l0 and permits rapid and effective cylindrical layering andleveling of the molten 6 and liquid 3 materials. The cylindricallylayered ring 26 of molten material substanially solidifies to a solidtube 14 on the ring 22 of heat-conducting liquid mold material whichflows axially down the bore of the centrifugal tube caster dowards theexit end 9 and becomes a thin ring 24 of restricted flow (in accordancewith Method 1 for creating a back-pressure on the liquid mold material3) as it passes over the exit orifice weir 25 of axially extendedsurface area, adjacent to the periphery of the solidified tube 14, whichcreates a line pressure drop along its length (in accordance with Method2 detailed in this disclosure) which accentuates the back-pressure onthe liquid mold material 3 to the extent that the DB. of the cast tubeis maintained less than the ID. of the exit orifice weir. The rotatingsolid tube 14 exits axially from the centrifuge for cut-off, capping,seal crimping, or continuous collapse deformation as desired, while theliquid mold material 22 spins off as a tangential stream 15 into asuitable annular catch-ring 83 and is recirculated by conventional meansnot shown. These means, along with the rotational mechanisms and spraycooling method, are indicated but are not detailed since they are a partof the prior art and well understood by those versed in such techniques.l-Iere also, the hot zone, as 10, may be extended in length so that slowcooling of the molten tube material can be accomplished. In this manner,when desired, accentuated gravity segregation results (as delta ferritebeing centrifuged towards the outside surface of a mild steel tube whichis later to be converted to automotive sheet steel).

In the mechanism of FIG. 5, it is preferred to use a means of any type(not shown) to control the rate of exit of the cast tube out of the exitorifice. In this manner, the casting machine can be of any convenientlength, and the axial movement of the exiting tube is slowed down to anextent that guarantees the CD. of the tube being less than the LB. ofthe exit orifice at a point considerably to the rear of the exit. Inthis manner, the cast tube floats out of the bore on a lining of liquidmold material and without danger of jamming. However, the length of themachine is not unduly restricted since casting rates can be increasedmerely by lengthening its bore.

FIG. 6 is illustrative of a vacuum seal means at the entrance end 2 of aliquid mold continuous centrifugal tube casting machine wherein a solidnon-rotating disc has its periphery 31 immersed into the liquid moldmaterial 3 which is contained in the annular rotating trough l9. Passingthrough and vacuum sealed to the non-rotating end plate 30 are theliquid mold conduit 5, the molten tube material conduit 7, a dry inertgas purge tube 32, and a vacuum suction outlet 33. The purge tube 32 (orother sealed entrance conduit) may be used as a plasma torch entrancefor the purpose of heating up the refractory part 4 prior to start-up.In this instance, the inert gas (as helium, argon, nitrogen, etc.) fromthe plasma torch also acts as an initial purge of the centrifuge cavity,and the torch melts down the starter blank which has solidified withinthe bore of the centrifuge from the prior shut-down operation. Thesuction tube 33 is fairly large and connects to a vacuum pumping system(not shown) so that the interior cavity of the centrifuge can becontinuously pumped down to any desired vacuum.

Exterior to the exit end 9 of the centrifugal casting machine is a setof opposed forging rolls 34 and 35 which travel axially and insynchronism with exiting tube 14. At the same axial location and atright angles to the plane between the axes of the forging rolls (34 and35) are two opposed banks of burners (as, not shown, plasma torches)which maintain the heat of the exiting tube 14, or bring it to a desiredforge welding temperature. These forging rolls 34 and 35 movesynchronously and axially along with the hot tube and gradually cometogether with sufficient force to collapse a small portion of the tube(as a two-foot length) to a solid round having a forge welded interior36 which is vacuum tight. Such collapsed sections of the tube can be asfar apart as desired (as every 300 feet of solid tube length) andprovide the vacuum seal to the tube at the exit end of the centrifugalcaster. Further on, and after another seal has been so forge-closed, thesolid section 36 can be cut off at its mid-length 37 for removal of thediscrete length of vacuum sealed sausage-like tube lengths, for use aspreviously described. It can be appreciated that other conventionalmeans, as swaging, flat-crimping, etc., can be used to form the discretecollapsed section for vacuum closure, beyond the exit end 9, of the hottube. Also, the axial travel of the sealing rolls (34 and 35) can beextended (as to 300 feet) so that they act as pull-out grips for thetube so cast.

FIG. 6A is a partial sectional axial view of another configuration ofthe entrance end 2 vacuum seal means wherein the stationary seal disc 30is peripherally immersed in an annular trough 40 of a low melting liquidmetal, such as Woods metal or molten tin. It has the advantage ofpermitting the seal to be at a lower temperature and obviates oxidationlosses of the seal fluid. In this case, both the molten tube materialand the liquid mold material 3 are subjected to the internal vacuum atthe entrance end 2.

FIG. 6B is representative of another such configuration wherein theannular seal trough 40 is intermediate between the molten tube materialtrough 16 and the liquid mold material trough 19. By this means, theliquid mold material is not subject to the internal vacuum, but to theambient atmospheric pressure, and this helps to raise the level(decrease the CD. of the molten tube 26) of the liquid mold materialwithin the bore of the caster.

FIG. 6C is another variation of the vacuum seal means at the entranceend 2 wherein the method of FIG. 6B is further enhanced by use ofanother end plate 41, exterior to the end plate 30, which isperipherally immersed into an annular rotating trough of liquid sealingmetal 42. This system permits the liquid mold material 3 in annulartrough 19 to be pressurized via inert gas tube 38 while the interiorcavity of the centrifuge is subjected to vacuum. The system of FIG. 6Cis even more effective in reducing the CD. of the molten tube 26 to thedesired size.

FIG. 7 is illustrative of a means for applying a positive pressure ofinert gas to the outside of the solidified tube 14 at the exit end 9 ofthe continuous centrifugal tube caster. The inert gas 50 is intoducedinto the end closure 51 via the high pressure gas tube 52, and thepressurized gas 50 acts on the liquid mold material 3 at the point oftangential spin-off so as to produce a greater than normal back-pressureon. the liquid mold lining 22 within the centrifuge. This back-pressure(Method 4) causes the heat-extracting ring of liquid mold material at 22to push inwardly and decrease the CD. of the molten metal tube to anydesired limit.

The end closure 51 is sealed at the annular area 53 (exterior to theexit end 9 of the centrifugal tube caster) by means of an iris ring ofcarbon or graphite blocks 54 of layered blades (other structuralmaterials, such as ceramics or metals, can be used, but arenonpreferred), which are contained within the annular holding rings 55.An annular pressure cavity 57 is behind the iris blocks 54 so that, bypressurizing this annular cavity 57 by means of the high pressure inertgas line 58, the iris blocks 54 are forced against the OD. area 53 ofthe centrifuge to form a pressure seal. Alternately, the seal at thearea 53 can be of the liquid metal type as designated by trough 40 ofFIG. 6A.

A similar inert gas pressure seal exists at area 60 on the opposite.side of the end-closure 51 so as to prevent undue gas leakage around thetube periphery. This iris of carbon 'ploughs or blocks 61 also act asscrapers to remove any excess liquid mold material from the periphery ofthe tube. Alternately, a carbon iris block 62 can be used which has amultiplicity of small radial holes 63 leading from the annular pressurecavity 64 to the ID. of the blocks 62 at the area 60. Passage of highpressure inert gas (as nitrogen) through the holes 63 onto the peripheryof the tube 14 at area 60 causes a gas bearing action which wipes backany excess liquid mold material into the closure 51 and, at the sametime, maintains the desired inert gas pressure therein. As a stillfurther alternate, the pressure cavity 64 may be pressurized withrelatively cool liquid mold material 3 so that a liquid bearing seal isformed. This alternate would only be used where a maximum amount ofliquid rnold material was desired as an exterior coating to the tube soproduced.

The iris ring of layered and overlapping carbon blades act as a variablediameter pressure containing seal whichwill conform to any inadvertentchanges in the tube diameter. More importantly, at startup, thesolidified starter tube is at a relatively low temperature and has acorrespondingly smaller diameter than when in the hot casting state. Inthis instance, the iris seal permits startup without requiring asubstantial increase in the flow of liquid mold material to maintain themelted tube at the desired diameter.

Referring to FIG. 8, this partial axial sectional view of the entranceend 2 of the centrifugal continuous tube caster illustrates s simplifiedmeans of sluicing the molten tube material 6 onto the ID. surface of theaxially flowing ring 22 of liquid mold material 3. In FIG. 8, theperipheral flow-holes for the liquid mold material 3 terminatedownstream at a point 27, and the refractory part 4 continues downstreamand tapers to an annular feather edge at point 28. At point 28, theaxial flowing annular rings of molten tube material 26 and of liquidmold material 22 come into heat exchange contact with a layered laminarflow. The shelf 17 of the refractory part 4 acts as a hot zone forlayering and leveling of the molten metal 26. This is the simplesttechnique, but not the preferred one, for introducing the I6 molten tubematerial layer 26 onto the liquid mold layer 22.

FIG. 8A represents an improvement of the method for sluicing the moltenring of axially flowing tube material 26 onto the axially flowing ringof cool liquid mold material 22 via an interposed thin ring I8 ofaxially flowing hot liquid mold material which is introduced onto theshelf 17 of the refractory part 4 by way of the small inclinedflow-holes 23 to produce a hot zone or extended hot zone 10 as desired.

The preferred technique for producing a hot or extended hot zone 10, andfor bringing the axially flowing annular streams of molten tube materialand hot and cool liquid mold materials into laminar contact, isillustrated in FIG. 8B. In this technique, an annular trough 16 isfilled with a small flow of liquid mold material 3 by way of the smallducts 23 which lead from the liquid mold trough 19 to the bottom of themolten metal trough 16 from whence it flows internal to the ledge 17 ofthe refractory part 4 as a hot, relatively thin lining which supportsthe molten metal ring 26. The molten tube material 6 pours onto thesurface of the liquid mold material, which fills the trough l6, andheats the liquid mold material to a temperature above the melting pointof the tube material. The annular trough 16 serves the purpose ofdecreasing the impact of the mo]- ten tube material input 6 and ofcreating an effective layering and leveling zone even prior to thedownstream hot zone represented by the relatively thin hot liquid moldlining 18. At the downstream sharp edge 28 of the refractory part 4, thehot liquid mold lining l8 continues downstream for a short distance andacts as a buffer between the axially flowing cool liquid mold ring 22and the molten tube material 26 and prevents too rapid chilling of thetube. It is preferred that all three annular rings (the molten tubematerial ring 26, the hot liquid mold 18, and the cooler liquid mold 22)have an approximately synchronized axial flow rate at the point 29 wheresolidifcation of the molten tube metal begins.

All of the systems illustrated in FIGS. 8 to 88 can be used inconjunction with the entrance end vacuum seal means of FIGS. 6 to 6C.

FIGS. 9, 9A and B illustrate various means by which the pressurized (asby any means such as a pump or overhead reservoir, not shown) liquidmold material 3 can be introduced at the exit end 9, or at any pointintermediate between the exit and starting ends, of the casting machine.

FIG. 9 shows an annular trough assembly 151 which is similar to assembly51 shown in FIG. 7, with the exception that, instead of a pressurizedgas being introduced into the assembly 51 to create a back-prssure onthe liquid mold material 22 lining the bore of the casting machine, theliquid mold material itself is introduced under pressure and fills theassembly trough 151. The walls of the trough assembly 151 are sealed tothe exterior surface of the rotating mold wall 13 at point 53 and to theexterior surface of the rotating solid tube 14 at point by means of irisrings 157 as shown in FIG. 7. The makeup of these iris seal rings areshown in greater detail in FIG. 4 of my US. Pat. No. 3,605,859 issuedSept. 20, 1971.

' Whereas the non-rotating iris seal rings at 53 and 60 are shown asbeing forced against the rotating members 13 and 14 by means of a liquidor gas pressure at their back faces, it is preferred to spring-load theiris rings since the liquid mold material at the point of introductioninto the casting machine is relatively cool. FIG. 9 shows the basicassembly 151.

FIG. 9A shows a series of such assemblies 152, 153, etc. on either sideof the basic assembly 151. The other contiguous assemblies 152 and 154,which is not shown, may be pressurized with liquid mold material 3 at alower pressure than that in assembly 151 by way of maintaining thepressure therein. A multiplicity of such sequential assemblies extendedtowards either the exit or entrance end of the apparatus can be used ifdesired.

FIG. 9B shows a pressurizing assembly 159 which can be located at anyconvenient point or points between the exit and entrance ends of thecasting machine. The assembly 159 is similar to the basic pressurizingassembly 151, except that the iris seal rings 157 are forced against thesolid mold wall 13 at points 161 and 162 of either side of an annularring of apertures 160 which provide entrance for the pressurized liquidmold material in assembly 159, to form the liquid mold lining 22.

In the arrangements of FIGS. 9-98, the length of the basic assembly 151can be greatly extended to give greater heat extracting contact betweenthe cast tube 14 and the liquid mold material 3. More than this, anoverflow standpipe or weir, not shown, can be located at the top ofassembly 151 to create an overflow head of liquid mold material. In thismanner, the heated liquid mold material in assembly 151 is continuallyreplaced with cooler liquid mold material for more efficient heatextraction. Likewise, the assemblies 152, 153, etc. can be extended ormultiplied for the same purpose.

In the case where such an assembly (as 151) partly encompasses theoutgoing tube 26 in its still molten state, it is preferred to introducethe liquid mold material tangentially and in the same direction as therotation of the molten tube and in general synchronization with itsrotational speed to create as little disturbance to the molten materialas possible.

FIG. 10 shows the design of the casting machine at the entrance or tubecasting end, in its simplest form, when the liquid mold material 3 isback-pressured (Method by any of the arrangements shown in FIGS. 9. Inthis case, the liquid mold material has no gross axial flow, but ismerely pressurized to a liquid level that will float the cast tube outof the bore of the casting machine.

FIG. A shows a similar, non-axially flowing, entrance end design wherebythe level of the liquid mold material may be observed in annular trough19.

FIG. 10B is similar to FIG. 10A except that the trough 19 has anoverflow weir 1 that permits the liquid mold material to overflow intoan annular collecting trough, not shown. By this arrangement, the lining22 of liquid mold material 3 can flow axially countercurrent to themovement of the cast tube for better heat extraction. The arrangement ofFIGS. 10 and 10A provide a natural hot zone of liquid mold material atthe starting (tube casting) end of the casting machine. However, thearrangement of FIG. 10B is greatly preferred since, with a non-flowinglining 22, the hot zone can extend axially towards the exit end of thecasting machine to a greater extent than desired due to centrifugallayering of the very hot (and therefore lighter) liquid mold materialadjacent to the periphery of the tube material.

FIG. 11 shows an arrangement whereby the liquid mold material 3 can beforced into the casting machine at its starting end by means of apressurizing assembly 151, the walls of which locate against the solidmold wall 11 and an inward cylindrical extension thereof 93. Thearrangement of FIG. 11 shows a small amount of liquid mold materialflowing into the hot zone 10 by way of a plurality of small vents 23,while the preponderance of the cooling liquid mold material 3 comes intocontact with the molten tube at a downstream point 21. The small vents23 can be dispensed with, if desired, to create a non-axially flowinglayer 18 of liquid mold material in the hot zone 10. A highertemperature hot zone will result.

FIG. 12 shows a simplified apparatus for the continuous centrifugalcasting of tube vertically downwards on a lining of liquid moldmaterial. The apparatus of FIG. 12 is illustrative only since all of thedesigned machines shown for continuous horizontal tube casting can beadapted to cast vertically downwards or upwards by simple designchanges, such as those shown in FIG. 1].

Horizontal casting has by far the greater advantage inasmuch as themolten tube material and the solid tube produced can be handled at floorlevel, and much longer tubes can be produced with far greater facility.

In FIG. 12, a relatively small amount of liquid mold material 3 isintroduced into the apparatus by way of spout 5 and annular trough 19.The liquid mold material then flows, by way ofa plurality of small holes10], onto the bore of the casting machine and forms a thin layer 22 ofcentrifuged liquid mold material between the solid mold wall 13 and thecentrifuged cylinder of molten tube material 6. The molten tube material6 is introduced through the entrance opening 2 by way of spout 7 andflows tangentially, and in the same direction as the rotating apparatus,to create a smoothing action. Layering and smoothing of the molten tubematerial into a cylinder is also facilitated by use of a hot zone 10which is maintained as short as possible for said layering in order tocongeal the molten cylinder to tube prior to gravity accelerating themolten layer of tube material downwards. The apparatus can use aninternal offset and tapered rotating core mold, now shown, asillustrated in British Patent No. 984,053 issued in 1963, to accomplishthis same purpose. External to the solid walls 13 of the apparatus are aplurality of spray nozzles, not shown, similar to those of FIG. 5, forheat extraction. At the exit end 9 of the casting apparatus is anannular trough which may be similar to annular trough 83 of FIG. 5, butis shown in FIG. 12 as having annular iris seals as depicted in FIGS. 9.External to the exit end 9 of the apparatus are a plurality of rolls,102 and 103 being shown, which control the exit rate of the cast tubeand prevent it from falling out of the bore of the casting machine. Theplurality of rolls, 102, etc., are attached to a frame, not shown, whichrotates in unison with the rotating cast tube, and the rolls arepowered, by means of a suitable gear train (not shown) to give thedesired rate of controlled exit of the cast tube 14.

FIG. 13 shows a simplified apparatus for the continuous centrifugalcasting of tube in a vertically upwards direction using a lining ofliquid mold material 3. In FIG. 13, the external casing 11 of therotating mold may be supported on suitable bearings, not shown, and berotated by a powered gear drive, not shown, about its vertical axis 1.The liquid mold material 3 is introduced into an annular trough, whichfaces upwards, by means of the inverted .I-shaped spout 5. The moltencasting material 6 is introduced into the same trough by way of inverted.I-shaped spout 7 and is fed from a holding reservoir 105 having acontrolled liquid level which assures a constant rate of introduction ofthe molten material 6. Similarly, the liquid mold material 3 has acontrolled rate of introduction from a similar reservoir, not shown.Both the liquid mold material 3 and the molten tube material 6 form anaccentuated parabolic curve upwardly within the solid cylindricalencasement 13 and the distance these materials move upwardly is afunction of the mold diameter and the rotational speed of the castingapparatus. It is preferred to operate at high G (centrifugalacceleration) values since this forces the liquid and molten materialsfurther up the bore and increases the length of wall over whichsolidification of the molten tube material can take place. External tothis length of wall, where solidification takes place, is a plurality ofperipherally spaced spray nozzles 104 for heat extraction throughtheencasing solid wall 13 and the lining of liquid mold material 22.

At the extreme upper point to where the liquid mold material 3 (which isintroduced at the bottom or starting end of the apparatus) ascends, is aplurality of annularly spaced holes 171 through which'the liquid moldmaterial can overflow. An annular trough assembly, not shown, havingiris seals, not shown, which locate at points 172 and 173, collects theoverflow and returns it, via a conventional heat exchanger, pump andliquid mold material rejuvenation system, to the starting reservoir.Upwards from the overflow point 171, may be located other annular ringsof holes, as at 174, for further introduction of cooler liquid moldmaterial by way of system 159 as shown in FIG. 9B; and upwardly from 159would be located another annular overflow trough system at 83. Such anextended system adds to the heat extraction capabilities of theapparatus and permits a faster casting rate. Beyond the upper end 9 ofthe solid wall encasement is a plurality of peripherally spaced pull-outrolls, 102 and 103 of which are shown, which rotate'on a suitable frame,not shown, in synchronism with the exiting solid cast tube 14 and whichare suitably geared to give a controlled axial rate of extraction to thetube. Upwardly from the pull-out rolls such as 102, 103 is a travelingcut-off assembly 180 which severs the tube after a desired length hasbeen produced. The severed lengths of cast tube are handled and removedby any suitable known type of mechanism, not shown.

The above apparatus is a simplified version and is readily extended toencompass the more complex means of the overall invention. For example,the liquid mold material may be introduced through the molten castingmaterial in the form of solid shot, or it may be introduced underneaththe molten casting material by way of an upwardly'facing annular troughimmediately inwards (diametrically) from the shown main trough. Such aninward trough, similar to trough 19 of FIG. 12, would connect to themain trough by means of a plurality of small holes through therefractory body 4. Also, the stationary end plate 30 may besaucer-shaped with the periphery thereof immersed in a centrifugallymaintained annular trough of liquid sealant. Such an apparatus is shownin FIG. of my US. Pat. No. 3,605,859, except that the rotating sealtrough would be attached to the bottom end of the casting machine. Bysuch a non-rotating bottom plate seal and by suitably sealing the upperend of the cast tube 14, a partial vacuum can be drawn, if desired, onthe tubes interior, with attendant advantages.

It can be seen that a great many variations of design can be adapted touse the basic principles of the present invention which are, primarily,the adjustment of the cast tube s diameter in its readily changeablemolten state, the floating of the molten tube inwardly to a desireddecreased diameter, and the floating of the tube out of the exit orificewithout restrictive contact therewith for more ready egress from thecasting machine. As an illustrative example, the design of Bathom, US.Pat. No. 3,367,400, could be made more effective by introducing a liquidmold material, such as liquid lead, through the machine instead of hisencompassing centrifuged cylinder of water coolant. Centrifuged liquidlead does not exhibit boiling with its attendant problems. Further, asmall amount of liquid mold material could be added to the moltencasting material to form a lining on the dry wall portions of theHathorn casting apparatus; or, the dry wall mold portions could be madeof a refractory material and the cylinder of casting material could comeinto contact with the main body of liquid mold material downstream andin a molten state. All such design variations are within the province ofthe apparatus of this invention.

Further, whereas point 9 (as shown in FIGS. 4, 5 and 6) is the end ofthe solid cylinderical shell encasing the liquid mold material and (inthe simplified versions of the apparatus) generally indicates the exitend of the apparatus, the invention pertains to a liquid wall mold. Assuch, the exit end of the apparatus is where the tube makes its finaldeparture from the liquid mold material. In the more sophisticatedversions of the apparatus, (as shown in FIGS. 9, 9A and 13) thisdeparture occurs at the end of the multiple non-rotating assemblies 151,152, etc. and exit port assemblies 83 (which may be utilized in anyseries such as 151, 83, 152, 83 etc. if so desired) and, in such cases,the exit end of the apparatus would be considerably further away frompoint 9 than in the simplified apparatus cases.

It should also be noted that the iris seals 61, 62 and 157, as shown inFIGS. 7, 9 and 9A are of a variable [.D. since they can expand orcontract against the CD. of the solid (or solidified on the OD.) exitingcast tube 14. As such, the CD. of the cast tube is always not greaterthan the ID. of the exit orifice.

Rejuvenation of the Liquid Mold Material:

Regardless of the use of internal and external inert atmospheres, theliquid mold (whether of lead, tin, or leadtin alloys) will graduallybuild up an oxide content which, being lighter than the liquid moldmaterial, will centrifuge to the 1D. surface of the liquid mold andadhere to the CD. of the metal tube being cast. Normally, thisconcentration is not large enough to cause problems but, at heavierconcentrations, it can cause excessive drag-out of the liquid moldmaterial and, in extreme cases, clogging of the conduits and flow-holesof the system. This can be corrected either by continuous or occasionalpassage of the liquid mold material through abath of molten cyanides (asthose of sodium, potassium or barium, or mixtures thereof). By suchtreatment, an oxidation-reduction reaction takes place that produces areduced liquid mold material that is completely rejuvenated(oxide-free). Carbon or graphite mold linings also rejuvenate the liquidmold mate-

1. Apparatus for continuously casting tubing, comprising: asubstantially cylindrical mold container having an inlet at one end andan outlet at its other end of greater inside diameter than the outsidediameter of the said tubing when at said outlet; means for rotating saidcontainer generally about its longitudinal axis to produce substantialcentrifugal force; means to introduce a liquid lining substance intosaid container to an inside diameter when at said outlet less than theaforementioned inside diameter of said outlet; means to inject moltencasting material into said inlet, said lining substance having a lowermelting point and greater density than said casting material, saidlining substance assuming the form of a liquid lining and said moltencasting material assuming the form of a congealed tube of smallerdiameter than and flOating through said outlet on said lining inresponse to said centrifugal force, the wall thickness of said tubebeing determined by the relation between the rate of injection of saidmaterial and the rate of exit of said tube floating from said outlet;diameter control means to control the outside diameter of said tubewhile molten so that said tube congeals substantially back of saidoutlet to a diameter no greater than the diameter of said outlet, andmeans external of said outlet and for controlling the rate of exit ofthe congealed tube in an unrestricted manner from said outlet.
 2. Theapparatus of claim 1, wherein the gas inside said tube is substantiallyat atmospheric pressure and said diameter control means comprisespressurizing means to apply a higher pressure to said liquid lining toconstrict said tube.
 3. The apparatus of claim 2, wherein saidpressurizing means comprises a restricted annular gap between said tubeand said outlet, said gap being dimensioned to create a back pressure insaid liquid lining to constrict said tube.
 4. The apparatus of claim 2,wherein said pressurizing means comprises an axial extension of theoutlet bore end portion of said mold container, said extension beingdimensioned to create sufficient fluid resistance to the outward flow ofsaid liquid lining substance to create a substantial back pressuretherein, said pressure constricting said tube.
 5. The apparatus of claim2, wherein said pressurizing means comprises means to apply a gaspressure to said liquid mold material, said pressure being transmittedby said material to said tube to constrict it.
 6. Apparatus forcentrifugal casting of tubing from molten casting material, comprising:a substantially cylindrical mold container having an inlet adjacent oneend and an outlet exit orifice adjacent its other end; means forrotating said mold container about its longitudinal axis to producesubstantial centrifugal force therein; means for introducing a liquidlining substance of lower melting point and higher density than saidcasting material into said mold, said substance assuming the form of aliquid lining in response to said centrifugal force; means for injectingsaid molten casting material into said inlet, said material assuming theform of a tube in response to said centrifugal force and floating onsaid lining and being cooled and congealed thereby to a state in therange of solid to semi-solid; means for controlling the rate of exit ofsaid tube from said outlet; and control means to control the diameter ofsaid tube in its molten state to restrict its outside diameter, aftercongealing, to a value no greater than the inside diameter of saidoutlet, to permit free exit therethrough, said control means comprisinga seal at said inlet, a closure of said tube after its exit from saidoutlet, and means for maintaining a pressure differential between theinterior and exterior of said tube, said pressure differential being ofsufficient magnitude to constrict the molten tube to a diametersubstantially below the normal limit imposed by Archimedes'' Principle.7. The apparatus of claim 6, wherein said control means furthercomprises: internal pressure means for creating an internal gas pressurelower than atmospheric inside said tube in said mold, and externalpressure means to apply an external pressure greater than said internalpressure to the exterior of said tube.
 8. The apparatus of claim 7,wherein said external pressure is substantially atmospheric.
 9. Theapparatus of claim 7, wherein said external pressure means comprises arestricted annular gap between the outside of the exiting tube in itscongealed state and the inside of said exit orifice, said gap creating aback pressure in said liquid lining to constrict said tube.
 10. Theapparatus of claim 7, wherein said external pressure means comprises anaxial extension of the bore of the apparatus to a length suffIcient tocreate substantial frictional resistance to flow to produce a linepressure drop in said liquid mold lining, said drop creating a backpressure in said lining to constrict said tube.
 11. The apparatus ofclaim 7, wherein said external pressure means comprises means to apply apressure greater than atmospheric to the exterior of said tube after itsexit, to create a back pressure on said liquid lining to constrict saidtube inside said mold.
 12. The apparatus of claim 7, wherein saidexternal pressure means comprises means to apply a pressure greater thanatmospheric at the inlet end of said mold container.
 13. Apparatus forcontinuous centrifugal casting of tubing from molten casting material,comprising: a mold container having a bulbous cavity, an inlet at oneend, and an exit orifice at its opposite end; means for rotating saidmold container to produce substantial centrifugal force; a partialfilling in said cavity of a liquid lining substance of greater densityand lower melting temperature than said casting material, said fillingextending radially inward to a diameter substantially equal to that ofsaid exit orifice, but not overflowing it, and forming a liquid mold insaid container in response to said centrifugal force; means forinjecting said molten casting material into said inlet inside saidlining, said material assuming the form of a tube under the influence ofsaid centrifugal force, said material being cooled toward solidificationby said lining; diameter control means applying pressure to said liquidlining to reduce the diameter of said tube in its molten statesubstantially below the normal limit imposed by Archimedes'' principle,to aid in its clearance, when congealed, through said exit orifice; andmeans for controlled exit of said tube when congealed to make itsdiameter less than that of said orifice to permit free passagetherethrough.
 14. Apparatus as in claim 13, wherein: said diametercontrol means comprises means to apply a gas pressure greater thanatmospheric to the exterior of said tube after its exit and to said exitorifice, said pressure being transmitted to said liquid lining toconstrict said tube in said mold.
 15. Apparatus for the continuouscentrifugal casting of tube, comprising: a rotatable mold containerhaving an entrance end and an exit orifice; means to introduce a moltentube material into said entrance end; means to introduce a liquid moldmaterial into said mold container to act as a liquid mold therein; meansto create a positive gas pressure differential between the exterior ofthe molten tube at said exit orifice and the interior of the tube beingcast, said differential being sufficient to substantially constrict thediameter of said molten tube to permit it to exit from said orificewhile molten in a stable state condition without contacting the solidportion of said orifice; means for rapidly cooling said molten tubeafter exit and prior to any substantial disturbance of said stable statecondition, to at least a semi-solidified state; and means for causingcontrolled exit of said tube.
 16. Apparatus for the continuouscontrifugal casting of tubing from selected molten casting material,comprising: a generally cylindrical mold container having an inlet atone end and an outlet at its other end, both disposed on itslongitudinal axis; means for rotating said container about said axis toproduce substantial centrifugal force therein; means for injecting intosaid container a liquid lining substance of higher density and lowermelting point than said casting material, said substance assuming theform of a liquid lining in response to said centrifugal force; means forinjecting said molten casting material into said inlet internally ofsaid liquid lining, said material assuming under said force the form ofa molten tube floating on the inner surface of said lining; said moltencasting material bEing cooled toward solidification and progressingaxially toward said outlet; an axially-extending cylindrical sleeve ofrefractory, thermally-insulating material in said container coaxialtherewith and extending from the vicinity of said inlet toward saidoutlet and forming an extended hot zone within said mold, said castingmaterial remaining substantially molten while in con-tact with saidsleeve, said hot zone being extended, beyond the length required merelyfor layering and levelling of said molten casting material, far enoughto permit gravity segregation in said material sufficient tosubstantially modify the physical properties of said tubing; and meansfor controlling the rate of exit of said tubing from said outlet. 17.Apparatus as in claim 16, wherein: said axially-extending cylindricalsleeve is made of pyrolitically-deposited material so oriented that itis relatively thermally-insulating in the radial direction and arelatively good heat conductor in the axial direction.
 18. The apparatusof claim 16, wherein: the inner surface of said sleeve has a relativelythin layer of said liquid lining substance to provide a surface on whichsaid molten casting material floats.
 19. Apparatus for the continuouscentrifugal casting of tubing from selected molten casting material,comprising: a tubular mold container having an inlet at one end and anoutlet at the other end, both disposed on a longitudinal axis of saidcontainer; means to inject into said container a liquid lining substanceof higher density and lower melting temperature than said castingmaterial; means for rotating said container to cause said liningsubstance to assume the form of a tubular liquid mold lining in responseto centrifugal force; means for injecting said molten casting materialinto said container through said inlet and internally of said lining,said material assuming the form of a tube under centrifugal force andbeing buoyantly supported and cooled toward solidification by saidlining; means for maintaining a desired pressure on the exterior of saidtubing after its exit from said outlet, comprising a non-rotatingannular pressurized enclosure enclosing both said tube and saidcontainer; encircling seal rings composed of seal-forming materialsealing said enclosure rotatably to said tubing and said container; andmeans for controlling the rate of exit of said tubing from said outlet.20. Apparatus as in claim 19, wherein said enclosure is substantiallyfilled with said liquid lining substance, said substance beingpressurized.
 21. The apparatus of claim 19, wherein: said seal rings arecomposed of a plurality of sickle-shaped blades pivoted at one end,pressure means forcing the inner surfaces of said blades in sealingcontact with the rotating surfaces; said blades being disposed inoverlapping relation around the circumference of said ring in the mannerof an iris diaphragm, whereby said blades maintain sealing contact overa range of inner diameters.
 22. The apparatus of claim 6, wherein: thegas inside said tube is at a pressure greater than atmospheric, and saiddiameter control means comprises differential pressure means forcreating a pressure differential between said lining and the inside ofsaid tube.
 23. The apparatus of claim 22, wherein: said differentialpressure means comprises a restricted annular gap between said tube andsaid outlet, said gap being dimensioned to create a back pressure insaid liquid lining to constrict said tube sufficiently to permit itsfree exit from said outlet.
 24. The apparatus of claim 22, wherein saiddifferential pressure means comprises an axial extension of an outletbore end portion of said mold container, said extension beingdimensioned to create sufficient resistance to the outward flow of saidliquid lining substance to create a substantial back pressure therein,said back pressure constricting said tube sufficiently to permit itsfree exit through said outlet.
 25. The apparatus of claim 22, whereinsaid differential pressure means comprises means to apply a pressuregreater than atmospheric to the exterior of the exiting tube to transmita back pressure of said liquid lining sufficient to constrict said tubein said mold to permit its free exit from said outlet.
 26. The apparatusof claim 22, wherein said differential pressure means comprises means toapply a pressure greater than atmospheric at the entrance end of saidmold container and a still greater pressure to said liquid lining,sufficient to constrict said tube enough to permit its free exit fromsaid outlet.
 27. The apparatus of claim 2, wherein said pressure meansis hydraulic.
 28. The apparatus of claim 6, wherein: the axis of saidmold is inclined at an angle to the horizontal, and further comprisingan annular trough at the inlet end of said mold and having a centralopening, and means to introduce said liquid lining substance and saidmolten casting material into said container, the hydrostatic pressure insaid liquid lining substance aiding to control the diameter of said tubeto facilitate its exit through said outlet.
 29. The apparatus of claim28, wherein said angle is substantially 90*.
 30. Apparatus as in claim6, wherein: said seal at said inlet comprises an annular trough with itsopening facing inwardly at the inlet end of said mold container androtating therewith, and containing a liquid sealing substance, and afixed seal disc with its periphery in said trough and sealed byimmersion in said sealing substance.
 31. The apparatus of claim 1,further comprising means for rejuvenating said liquid lining substance.32. The apparatus of claim 6, further comprising means for rejuvenatingsaid liquid lining substance.